The present invention relates to a function element, such as an LWI (lasing without inversion) laser element, capable of functioning without a population inversion based on the EIT (electro-magnetically induced transparency) in a medium such as a solid.
The entire contents of Japanese Patent Application No. 8-245365 filed on Sep. 17, 1996 are incorporated herein by reference.
Recently, studies are actively conducted for realizing the EIT by utilizing a semiconductor quantum well having comparatively discrete energy levels among various solids, and an impurity (such as A. Imamoglu et al., Opt. Lett. 19, 1744 (1994); P. J. Harshman et al., IEEE j. Quantum Electronics 30, 2297 (1994); D. Huang et al., J. Opt. Soc. Am. B11, 2297 (1994); Y. Zhu et al., Phys. Rev. A49, 4016 (1994)).
However, a significant EIT characteristic as shown in an atom gas has not been observed in a solid. The reason is that a quantum structure cannot be formed homogeneously in a semiconductor quantum well with the conventional element production technology so that energy levels vary widely.
Due to the inhomogeneausly broadened energy levels, the number of the quantum structures satisfying the EIT condition on detuning; .DELTA..omega..sub.1 =.DELTA..omega..sub.2 (.LAMBDA. type and V type excitation), or .DELTA..omega..sub.1 =-.DELTA..omega..sub.2 (.XI. type excitation) decreases, the EIT characteristics of the optical transition are small with respect to the atom gases. Therefore, large light modulation characteristics derived from the EIT, which are indispensable for an optical element, such as an LWI laser, have not been obtained in a semiconductor quantum well.
On the other hand, since transition with very little variation in the energy levels exist in the case of impurities, in particular, rare-earth ions or transition metal ions, the EIT conditions of detuning can be satisfied comparatively easily by the selection of such a transition.
However, the transition probability of the f--f transition of a rare-earth ion or the d--d transition of the transition metal ion is extremely small even if it is the allowed transition (the oscillator strength of a rare-earth ion or a transition metal ion is about 10.sup.-5 with respect to that of an usual allowed transition of .about.1).
Besides, the oscillator strength of such as forbidden transition of an impurity cannot be the ideal value, zero, but it is known that a transition having a value of about 10.sup.-5 to 10.sup.-8 exists.
Therefore, since it is difficult to have the contrast in the transition probability between the allowed transition and the forbidden transition, furthermore, the relaxation rates thereof do not vary widely, even if .LAMBDA. type three levels are set by the impurity energy level, it is difficult to satisfy the EIT conditions. Accordingly, large light modulation characteristics derived from the EIT have not been obtained in an impurity in a solid as in the semi-conductor quantum well.
As mentioned above, a function element such as an LWI laser based on the EIT utilizing a solid, and a light modulation element has a problem in that the transition probability and the relaxation rate of a solid do not coincide with the EIT conditions to disturb the realization thereof.